Sulfuric acid and sulfur trioxide may be formed by reacting a sulfur dioxide (SO.sub.2) containing gas with excess air over a suitable oxidation catalyst to form sulfur trioxide (SO.sub.3) by the reversible reaction: EQU SO.sub.2 +1/2O.sub.2 .fwdarw.SO.sub.3 +Heat
The sulfur dioxide can be supplied as such or formed by the combustion of sulfur with air or oxygen-enriched air. The formed sulfur trioxide is absorbed in water or in sulfuric acid.
In the evolution of processes for sulfuric acid manufacture, it became popular to use multiple contact and multiple absorption processes as disclosed, for instance, in U.S. Pat. Nos. 3,259,459 to Moeller, 3,362,786 to Burkhart, 3,525,587 to Browder, and 3,620,673 also to Browder, the disclosure in each is specifically incorporated herein by reference. The processes have varied dependant upon whether the supply or feed is sulfur or sulfur dioxide, and also in the route of passage of a sulfur dioxide/sulfur trioxide containing gas through a plurality of heat exchanges in route to or between a plurality of catalytic conversion stages. All rely on sulfur trioxide removal in a first stage of absorption following a first series of catalytic conversion stages, then passage of the residual gas stream through at least one final catalytic conversion stage to maximize conversion of sulfur dioxide to sulfur trioxide, followed by passing the resultant sulfur trioxide-containing gas stream through a final sulfur trioxide absorption stage before venting the remaining gas stream to the atmosphere. The gas passing to each catalytic stage is normally heated to its kindling temperature by heat exchange between the gases passing between catalytic conversion stages. In the case where sulfur dioxide is formed by combustion of sulfur, the heat of combustion may be used to heat the gas stream passing to the catalytic stage following an intermediate absorption stage to its kindling temperature. This process has been known over the past twenty years as a double contact/double absorption or DC/DA system.
U.S. Pat. No. 3,630,673 describes a basic flow scheme and operating parameters for a DC/DA plant based on a sulfur-burning process.
In a typical DC/DA sulfur-burning sulfuric acid plant, molten sulfur is burned with air to produce a gas stream containing about 9-12% by volume sulfur dioxide. The hot combustion products are usually passed through a waste heat boiler to reduce the sulfur burner exit temperature to the required feed temperature for a first catalytic conversion stage while producing high-pressure steam.
The first catalytic conversion stage consists of a plurality of catalytic beds in series, where the majority of the sulfur dioxide is converted to sulfur trioxide. Heat liberated by the reaction is removed between each bed of the first catalytic conversion stage and used to heat the gas passing from an intermediate absorption stage to a second catalytic conversion stage. The heat exchange ensures that a gas stream passing to any catalyst bed is at its kindling temperature and prevents overheating of the catalyst.
Following final conversion and cooling, the gas stream is sent to the second absorption stage which functions in a manner similar to the first absorption stage. After absorption of sulfur trioxide, the gas is passed through a final demister, then through a stack to the atmosphere.
Customarily, a three-stage converter with one final converter will produce an exit gas containing up to about 400 ppm by volume (ppmv) sulfur dioxide, sulfuric acid mist up to about 0.1 Kg per ton of produced sulfuric acid and up to about 400 ppmv oxides of nitrogen.
In countries with severe restrictions on sulfur dioxide emissions to the atmosphere, such as Japan, it is customary to use a two-stage converter with one final converter to produce an exit gas with up to about 1000 ppmv sulfur dioxide followed by use of an additional tail gas cleanup process to reduce the residual sulfur dioxide content in the exit gas to below 50 ppmv.
For a sulfur-burning sulfuric acid plant, heat enters the system from five sources and is removed by three main systems. Heat enters as heat of compression of the main air blower, heat of combustion of sulfur to sulfur dioxide, heat of reaction (oxidation) of sulfur dioxide to sulfur trioxide, heat of reaction of water with sulfur trioxide to form sulfuric acid, and heat of dilution of the formed acid with water. Steam is generated in waste heat boilers and lower level process heat is usually recovered by preheating boiler feed water in economizers or lost to the environment. The value of energy exported from a sulfuric acid plant is a significant factor in sulfuric acid plant economics.
While current sulfuric acid plants can achieve overall sulfur conversion efficiencies of up to 99.7%, there is continuing pressure to provide sulfuric acid plants with even higher efficiencies. The proposed routes, however, have been completely dependent on the use of catalytic converters and more complete removal of sulfur dioxide from the process gas by altering the equilibrium conversion, either by removing sulfur trioxide or by either increasing the oxygen partial pressure by either increasing the system operating pressure or using oxygen-enriched air or a combination of both. The practical and economic difficulties of building and operating air-based sulfuric acid plants operated at higher pressure are such that only a limited number of plants of this type have been built. No technology is available to reduce the nitrogen oxide formation and subsequent emission to the atmosphere.
European Patent Application 0 002 737 pertains to a non-catalytic process in which sulfur, sulfuric acid and/or ammonium sulfate and a recycle stream containing sulfur dioxide, sulfur trioxide and oxygen are combusted with oxygen at high temperatures and high pressures to form a product gas containing sulfur dioxide, sulfur trioxide and oxygen. Sulfur trioxide is condensed then distilled to eliminate remaining SO.sub.2. The inventors state that the process must be operated at a pressure between 500 and 5000 psig. At pressures below 500 psig the desired reaction will not be obtained because of the relatively low sulfur dioxide-oxygen molar ratio and relatively high concentration of sulfur trioxide in the feeds to the sulfur burner and at pressures more than 5000 psig formation of oxides of nitrogen would be enhanced. Moreover compressing oxygen rich gas stream above 500 psig is mechanically inefficient, expensive and a potentially hazardous operation.
It would be desirable to provide a process for sulfuric acid manufacture which is lower in equipment cost, at least competitive in processing cost, operates at relatively moderate pressures and which minimizes pollutants, such as sulfur dioxide, sulfuric acid mist, and nitrogen oxides to the environment. This is the purpose of the instant invention.